To further understand the indirect nature of our everyday thinking, we can say that, when we entertain a thought, we are entertaining a memory, and never thinking in direct contact with the object we have in mind. To think about a tree, we must first observe the tree, and by the time we begin to ‘think about it’ it is a memory of the perceptions through which we became aware of its reality. We cannot, in our normal discursive thought, grasp the essence of a thing: we can only deal in reminiscences, and these will always be partial: not only due to the imperfections of memory but to the limits of our perceptions themselves. We saw the tree but could only take in a caricature of it. Thus, all knowledge gained in this way is partial, piecemeal, and riddled with errors. To put it another way, our knowledge of the material world is much like the knowledge gained by means of a scrapbook filled with photos. We collect photos and we draw conclusions but we cannot directly perceive the thing itself via knowledge derived from empirical observation, from experience, and it is with this data that the rational faculty operates.
What, then, is the hope of really knowing things? If rational knowledge based on empirical observation were all man could achieve, then there would be no hope at all. But the reason does constitute the limits of human knowing.